The visible and thermal light curve of the large Kuiper belt object (50000) Quaoar (2401.12679v2)

Abstract: Recent stellar occultations have allowed accurate instantaneous size and apparent shape determinations of the large Kuiper belt object (50000)~Quaoar and the detection of two rings with spatially variable optical depths. In this paper we present new visible range light curve data of Quaoar from the Kepler/K2 mission, and thermal light curves at 100 and 160 $\mu$m obtained with Herschel/PACS. The K2 data provide a single-peaked period of 8.88 h, very close to the previously determined 8.84 h, and it favours an asymmetric double-peaked light curve with a 17.76 h period. We clearly detected a thermal light curve with relative amplitudes of $\sim$10% at 100 and at 160 $\mu$m. A detailed thermophysical modelling of the system shows that the measurements can be best fit with a triaxial ellipsoid shape, a volume-equivalent diameter of 1090 km, and axis ratios of a/b = 1.19 and b/c = 1.16. This shape matches the published occultation shape}, as well as visual and thermal light curve data. The radiometric size uncertainty remains relatively large ($\pm$40 km) as the ring and satellite contributions to the system-integrated flux densities are unknown. In the less likely case of negligible ring or satellite contributions, Quaoar would have a size above 1100 km and a thermal inertia $\leq$ 10 Jm$^{-2}$K$^{-1}$s$^{-1/2}$. A large and dark Weywot in combination with a possible ring contribution would lead to a size below 1080\,km in combination with a thermal inertia $\gtrsim$ 10 Jm$^{-2}$K$^{-1}$s$^{-1/2}$, notably higher than that of smaller Kuiper belt objects with similar albedo and colours. We find that Quaoar's density is in the range 1.67-1.77 g/cm$^3$, significantly lower than previous estimates. This density value closely matches the relationship observed between the size and density of the largest Kuiper belt objects.

• The paper refines Quaoar's rotation period using visible light curves, revealing an 8.88-hour period and suggesting an asymmetric double-peaked structure.
• The analysis of thermal light curves at 100 and 160 μm shows about a 10% amplitude variation, indicating notable surface or shape inhomogeneities.
• Thermophysical modeling determines Quaoar's size (≈1090 km diameter with axis ratios of 1.19 and 1.16) and revised density (1.67–1.77 g/cm³), aligning with trends in other KBOs.

Insights into the Light Curve Analysis of Kuiper Belt Object (50000) Quaoar

The paper under discussion presents an analysis of new visible and thermal light curve data for the Kuiper Belt object (KBO), Quaoar, utilizing observations from the Kepler/K2 mission and the Herschel Space Observatory. The primary objective of the research is to refine the understanding of Quaoar's physical characteristics, including its shape, size, and thermal properties, leveraging data from both visible and thermal spectrums.

Key Findings

1. Visible Light Curve Resolution: Quaoar's rotation period is measured with improved precision, indicating a primary single-peaked period of 8.88 hours, closely aligning with previously reported values. The analysis suggests a preference for an asymmetric double-peaked light curve with a period of 17.76 hours.
2. Thermal Emission Variability: The thermal light curve, observed at 100 and 160 micrometers, reveals relative amplitude variations of approximately 10%. This suggests notable rotational modulation in the thermal emission, indicative of surface inhomogeneities or shape-induced variabilities.
3. Shape and Size Determination: Thermophysical modeling, incorporating new data, supports a triaxial ellipsoid model for Quaoar. A volume-equivalent diameter of approximately 1090 km is proposed, with axis ratios a/b = 1.19 and b/c = 1.16. This model is consistent with both occultation-based shapes and observed light curve data.
4. Density Estimates: The derived mean densities, in the range of 1.67-1.77 g/cm³, are significantly lower than previous estimates, aligning more closely with the density-size relationship observed in other large Kuiper Belt objects. This adjustment lowers earlier, potentially overestimated, densities due to subsequent refinements in both the size and mass of the system.
5. Orbital Dynamics and System Complexity: Recent occultation observations have revealed a complex ring system around Quaoar. The existence of rings and a satellite, Weywot, complicates the interpretation of system-integrated observations by contributing to the total flux density.

Implications and Future Directions

The inferences drawn from the updated light curves and thermophysical models have substantial implications for our broader understanding of Kuiper Belt objects. The revised density estimates suggest that Quaoar and similar bodies might not deviate significantly from typical compositional models when accounting for volatiles and bulk structure. These findings affirm Quaoar's status as a transitional object in compositional and physical structures among KBOs, aligning with known characteristics of bodies like Sedna and Charon.

The detection of rings beyond Quaoar's classical Roche limit introduces intriguing questions regarding their formation mechanisms and stability, potentially offering analogs to planetary ring systems under different environmental conditions. Exploration of these systems, driven by advancing observational technologies and methodologies, could yield further insights.

Looking ahead, the integration of high-resolution spectroscopy, such as from the James Webb Space Telescope, could unravel more intricate details of the molecular and surface compositions of Quaoar. In addition, continued monitoring of its light curve could discern potential secular changes due to continued interactions with its satellite and rings.

In conclusion, the paper provides a comprehensive view of (50000) Quaoar, propelling the understanding of KBOs by delivering refined physical and dynamical attributes derived from recent light curve analyses. Such studies are critical milestones in developing astrogeophysical models that capture the evolving dynamics in the outer solar system.